HFProtection of Electronic Circuits

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HF-Protection ofElectronic Circuits

• Thomas Losert

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Overview

• Noise Sources (External, Internal)
• Basics about Shielding
• Examples
• Outlook

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Overview

• Noise Sources (External, Internal)
• Basics about Shielding
• Examples
• Outlook

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External Noise Sources

• External noise is received mostly through
  I/O-connectors or electromagnetic waves
• Shield I/O connectors (or even the cables) and/or
  use filters for blocking noise
• Use a star-topology for ground, i.e. connect all
  ground lines in a common point

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Internal Noise Sources

• Internal noise is caused mostly by signals with
  high frequency (e.g. Oscillator of
  Microcontroller) or high currents (e.g. switching
  power supply)
• Minimize length of circuit paths that could act
  as antennas for noise signals (e.g.,
  ground-planes)
• Maximize Distance to sensitive signals (e.g.
  measurement signals)
• Subdivide the system in (nested) System Zones
  and use filters for blocking noise at the
  boundaries
• Eliminate noise sources (e.g., no floating input
  pins)

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Overview

• Noise Sources (External, Internal)
• Basics about Shielding
• Examples
• Outlook

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Basics about Shielding

• Due to the skin effekt for higher frequencies a
  thinner shield is sufficient(e.g., conductive
  foil, tape, or paint)
• For higher frequencies (shorter wavelengths) the
  tolerable gap dimension in the shield decreases
• Noise should be eliminated at the source if
  possible (e.g., decoupling capacitors)
• Noise sensitive circuits should be separated from
  noise sources (e.g., ground planes, conductive
  material, or distance)

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Overview

• Noise Sources (External, Internal)
• Basics about Shielding
• Examples
• Outlook

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Example Decoupling Capacitors
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Example Ground Planes

• Current can only flow in loops
• Larger loops (bigger antennas) emit more noise
• Dedicated ground-planes help minimizing the
  effective area of the loop
• Ground-planes should not be used for routing of
  other signals
• Adding paths to the ground-plane require a
  detailed analysis of every track on the board
  (otherwise the ground-plane could be wasted)

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Example Shielded Circuit

• Metal case protects against undesired
  electro-magnetic waves
• feed-through capacitor for decoupling the power
  supply

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Example Subdivision of a System

• Further shielding could be achieved with
  compartments of conductive material
• For shielding of ventilation openings
  lattice-like panels are available

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Example Path to Ground

• Separate high noise/current lines and
  analog/sensitive signals

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Overview

• Noise Sources (External, Internal)
• Basics about Shielding
• Examples
• Outlook

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Outlook

• On Saturday, May 1st, 2004
• same time, same place
• PCB Design

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• Vielen Dank für Ihre Aufmerksamkeit !
• Noch Fragen?
• Thomas.Losert_at_tuwien.ac.at